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In addition, because the hosts are also evolving, they occasionally develop immune mechanisms that free them from parasites. However, the parasites generally return after a time, and the hosts and parasites show alternating patterns of advantage, as the parasites overcome the defenses of the hosts, and the hosts evolve new defenses.

Figure 9: Evolutionary race between hosts and parasites in a soup of the Tierra program.

Hosts, red, are very common. Parasites, yellow, have appeared but are still rare.

Hosts, are now rare because parasites have become very common. Immune hosts, blue, have appeared but are rare.

Immune hosts are increasing in frequency, separating the parasites into the top of memory.

Immune hosts now dominate memory, while parasites and susceptible hosts decline in frequency. The parasites will soon be driven to extinction. Each image represents a soup of 60,000 bytes, displayed as 60 bars of 1000 bytes each. Each individual creature is represented by a colored bar, colors correspond to genome size (e.g., red = 80, yellow = 45, blue = 79).

Thomas S.Ray
Mon Jul 15 15:51:28 JST 1996